An air bag is a passive device which is appropriately positioned in a vehicle, such as an automobile or a truck, and which is utilized to lessen the possibility of injury to the occupants in the event of a collision. Typically, a pressurized gas canister has a frangible valve mechanism which is ruptured in response to a signal transmitted to a thermitic device by a sensor so that gas may issue from the canister in order to inflate the bag.
Typically, the sensor is mounted to the frame of the vehicle approximately two feet or so rearward of the front end. As a result therefore, the front end of the vehicle must deform before the sensor is activated. Generally, approximately 10 milliseconds are required for the front end to deform to a significant extent during a collision. Subsequent to this, the thermitic device must rupture the valve and the bag must inflate. Collision sensing and bag inflation must be accomplished in no more than 30 milliseconds for the bag to adequately protect the user. The entire collision is usually completed in approximately 150 milliseconds.
Collision sensing is accomplished by reaction of the sensor to a sudden deceleration of the vehicle. The sensor must therefore be capable of distinguishing between rapid braking and a collision or else the bag may unnecessarily inflate, and possibly cause unintended damage. It is therefore readily apparent that vehicle collision sensors must be capable of acting with extreme speed and yet be able to distinguish between a collision and other forms of rapid deceleration.
Clarke, U.S. Pat. No. 3,177,312, discloses a slug type inertia switch. Clarke, particularly FIG. 2, discloses the utilization of a flat spring which closes the electric circuit between associated wires upon the slug being subjected to a minimum decelerating force. Clarke discloses that the spring may be a disc or a strip but fails to appreciate that an arcuate plastically deformed spring has significantly faster spring action. The switch speed of Clarke is limited to the marginal speed of the slug. The switch of Clarke is advantageously utilized with an artillery shell rather than a vehicle, and for this reason, switch speed is relatively unimportant. The relatively low reaction speed of Clarke is not appropriate for use in a vehicle. Furthermore, the spring of Clarke would have a tendency to resist movement of the slug and would not maintain positive contact without application of force.
From the above, it can be seen that a collision impact sensor which has rapid response and which distinguishes between a collision and other forms of rapid deceleration is necessary. The disclosed invention provides such a switch by mounting an arcuate conductive plastically deformed spring to a contact. The spring is concave in its first or open position and shifts or flips to a convex position wherein another contact is engaged upon the spring being subjected to force application by a movable mass. Preferably, the contact holding the spring has an aperture therethrough permitting the spring to snap through the plane of the contact for establishing electrical connection with the other contact. In this way, the conductive spring is an integral part of the circuit because current flows through the spring connecting the contacts. Means are also provided for resetting the switch and for biasing the spring so as to cause flexing whereby the speed of response is increased.